1. Field of the Invention
The present invention relates to a file system for storing files that are accessed by computers, and especially to a distributed file system that is realized by a plurality of server computers and a plurality of storage devices that are connected to network.
2. Related Background Art
In recent years, various business operations have become electronic with the advent of IT (Information Technology), and business operations that were conventionally processed with paper are coming to be processed with electronic files (hereinafter sometimes simply called “files”), which are electronic data. In the meantime, there must be a guarantee that documents such as agreements and transaction history that are used as evidence in audit or trials constitute original documents, and the original documents have not been altered.
At present, magnetic disks are the mainstream in terms of storage devices for storing electronic files, and the common mode is a mode in which a computer constructs a file system on magnetic disks and the computer stores electronic data as files in the file system. Due to the fact that it is easy to alter or erase files in the file storage method that uses magnetic disks and file systems, it has been difficult to guarantee that the files are original and have not been altered.
To solve this problem, according to a conventional technology, a storage device may calculate a hash value from the entirety of data stored in the storage device and uses the calculated hash value as an identifier for the data. Since the hash value would be different if the corresponding data is different, data can be guaranteed not to have been changed as long as users can access the data using the same hash value.
Although it is possible to guarantee that data has not been changed, a user must provide to a storage device a hash value that corresponds to the data in order for the user to obtain the data. Consequently, the user must manage the correlation between data stored and the hash value, which results in large management costs.
Further, in the conventional technology, since the user cannot assign a name of the user's choice to data and store the data in the storage device, once the user loses the hash value, the user cannot access the data.
Moreover, changed data is registered and managed in the storage device as data completely different from the original data, so that the relationship between the data after change and the original data is not managed by the storage device. As a result, if the user wishes to restore the changed data to the data before the change was made, the user must manage the relationship between the data before change and the data after change.